1. Field of the Invention
The present application is generally directed to a method and apparatus for operating at least two engine generators in parallel. More particularly, the present application is directed to a method and/or apparatus for operating a closed transition transfer switch (CTTS), such as those used in “make-before-break” emergency power applications, to enable parallel operation of at least two generators. However, aspects of the present application may be equally applicable in other scenarios as well.
2. Description of Related Art
A general diagram of a conventional engine generator paralleling system 10 is illustrated in FIG. 1. The system 10 may be used to power an emergency power bus 12 with three engine generators 13-15. As can be seen from FIG. 1, the system 10 comprises three separate circuit breakers 16-18 coupled to the power bus 12 and the generators 13-15. The power bus 12 is coupled to transfer switches 19-21. The transfer switches 19-21 are coupled to the power bus 12 at “emergency” positions 22-24. At “normal” positions 26-28, the transfer switches 19-21 are coupled to a utility bus 30, which is coupled to a utility power source 32 via a breaker 34. The transfer switches 19-21 are also coupled to load lugs 35-37. The utility power 32 supplies the load lugs 35-37 when the transfer switches 19-21 are each toggled respectively to the positions 26-28. Toggling generally refers to a transfer switch automatically changing between a “normal” and an “emergency” position and vice versa. For example, a transfer switch control panel (not shown) may automatically sense when the transfer switches 19-21 should be toggled. The master control 40 will then receive a signal from the control panel and accordingly toggle the transfer switches 19-21.
To supply power from the generators 13-15, the transfer switches 19-21 should each be toggled to the positions 22-24. The master controller 40 may be configured to shed lower priority loads or add higher priority loads to the emergency system, which may depend on available generator kW capacity. Typically, a generator system is designed so that the emergency power does not damage any loads. (see, for example, paragraph 6.3 of National Fire Protection Association standard 110).
The master controller 40 is also coupled to generator controllers 43-45, which are each coupled to the circuit breakers 16-18. When the utility power 32 fails, the master controller 40 initiates the startup of each of the generators 13-15. When at least one of the generators is at a suitable power, the master controller 40 closes at least one of the breakers 16-18 and at least one of the switches 19-21. The master controller 40 may provide more power to the power bus 12 by closing additional breakers. Before closing an additional breaker, the master controller may wait until the next generator is synchronized with the power bus 12 (i.e., the next generator matches the frequency and voltage phase angle of the other generators connected to the power bus).
Generally speaking, the system 10 and similar systems are usually custom designed and built, and due to complexity, they are comparatively more expensive than non-parallel systems. The costly nature of such systems may be attributed to installation and maintenance costs. For example, the paralleling circuit breakers 16-18, transfer switches 19-21, and controllers 43-45 are individual components, each of which must be installed and maintained separately, making paralleled systems more costly relative to typical non-paralleled systems
To reduce system costs, emergency power systems may include a pre-packaged solution that reduces design and installation costs. For example, one type of pre-packaged solution includes a transfer switch that automatically monitors incoming power. Certain anomalies such as voltage sags, brownouts or swells may cause internal circuitry (within the transfer switch or a controller) to command a generator startup and then a transfer to the emergency generator when the generator has the proper voltage and frequency. When utility power returns, or certain other anomalies have occurred for a set time, the transfer switch will then transfer back to utility power and command the generator to turn off after another specified amount of “cool down” time.
A “break-before-make” transfer switch breaks power contacts with one source of power before it makes contact with another. In one arrangement, such a transfer switch prevents backfeeding from an emergency generator back into the utility line. One example is an open transition Automatic Transfer Switch (ATS).
FIG. 2 shows one arrangement illustrating a pre-packaged system 50 that includes “break-before-make,” 2-position, ATSes 52, 54. The ATS 52 has an input 56 coupled to a generator 58 and an input 60 coupled to a generator 62. At a load output 64, the ATS 52 provides generator power to the ATS 54. In this arrangement, the ATS 52 may toggle between power supplied by the generators 58, 62 and the ATS 54 may toggle between the ATS 52 and a utility power source 64. However, when the ATS 54 is toggled to receive emergency power, both of the generators 58, 62 cannot provide power simultaneously to a load lug 66.